Effect of friction stir processing with different speed ratio factors on microstructure, hardness and superplasticity of Al-3Mg-0.1Sc-0.1Zr alloy was investigated. The results showed that with the increase of the speed ratio factor and heat input, the area of stir zone, the grain size and the proportion of high angle grain boundaries increased, and the dynamic recrystallization was more complete. But the peak value of hardness in stir zone decreased with increasing speed ratio factor and heat input. All the alloys processed with different speed ratio factors showed high strain rate superplasticity when they are tested at 475 ℃ with strain rate of 10_-2 ^s_-1^. Three types of true stress-true strain curves were observed during tensile tests. The optimal elongation of 2500% was achieved in the alloy processed with speed ratio factor of 4, which showed significant strain hardening before tensile fracture. This would improve the common softening loss of stress at the later stage of superplastic forming, implying a high engineering application value. The outstanding superplasticity was mainly attributed to equiaxed fine grains with high thermal stability and a high proportion of high angle grain boundaries. Based on the analysis of grain aspect ratio, cavities evolution and fracture profile, the dominant mechanism of superplastic deformation under all parameters was grain boundary sliding.